A Web Application (or web-based application) uses the web browser as the front end (client) for a Server Side? software application program - Middle Ware, which executes the Business Logic? on the Web Server or Application Server, perhaps with a Back End? DatabaseServer.

A Web Application allows the website to provide services to visitors, but these should not be confused with Web Services or Service Oriented Architectures, which provide services to other application programs.

The Web Application depends on some powerful capabilility in the browser like a Java applet, Javascript, or Mozilla XUL (XML-based User Interface Language).

Macromedia invented the term Rich Internet Application (RIA) to describe a Flash-based Internet application. They replace the web browser (and the HTTP protocol) with a proprietary client (working through UDP and other protocols) as the application development platform of choice in the future, with Flash the development Frame Work. They may hope to obsolete the web browser, as may even Microsoft, who has not updated their Internet Explorer? for many years.

Java applets, Macromedia Flash, and Mozilla XUL are the basis for more than one cross-platform Wysiwyg Editor? used in Content Management.

References:
Mozilla
Macromedia